1. Field of the Invention
The present invention relates generally to cement compositions utilized in the construction and repair of wells, and more particularly, to such cement compositions which have fine particle size and methods of their use.
2. Description of the Prior Arts
In cementing operations such as those carried out in constructing and repairing oil, gas and water wells, a hydraulic cement composition is prepared by mixing a hydraulic cement with water and other additives, the composition is placed into a subterranean zone to be cemented and allowed to set into a hard substantially impermeable mass therein.
After a well has been constructed and used in the recovery of fluids from or the introduction of fluids into subterranean formations, problems relating to the unwanted passage of fluids into or from subterranean formations or the well bore penetrating such formations sometimes occur. This unwanted passage of fluids can severely disrupt or terminate the operation of a well.
The unwanted passage of fluids referred to above ordinarily involves the flow of oil, gas or water through small openings such as holes or cracks in well casing; holes, cracks, voids or channels in the cement sheath in the annular space between the walls of the well bore and the well casing; very small spaces between the cement sheath and the exterior surface of the casing or the walls of the well bore; and permeable spaces in gravel packs and formations.
Heretofore, such openings have been plugged or attempted to be plugged using hydraulic cement compositions. However, when the openings are very small, it is difficult to cause a cement composition of relatively large particle size to enter the openings and set therein whereby the openings are plugged. If the cement composition utilized can not enter the openings, at best a bridge or patch may be formed over the openings to produce only a temporary termination of the unwanted fluid flow.
In primary cementing, i.e., the forming of the above mentioned cement sheath in the annular space between pipe disposed in a well bore and the walls of the well bore, a pumpable hydraulic cement composition is introduced into the annular space and the cement composition is permitted to set therein. The cement sheath provides physical support and positioning to the pipe in the well bore and prevents unwanted fluid migration between subterranean zones and formations penetrated by the well bore. If the hardened cement sheath between the pipe and walls of the well bore contains spaces such as voids, cracks or channels, the unwanted fluid migration between zones and formations can still take place. Other problems incidental to primary cementing involve the failure of the cement sheath to tightly bond to the exterior walls of the pipe or the interior of the well bore, the formation of fractures in or the breakdown of subterranean formations due to the hydrostatic pressure developed by the cement composition and other similar problems. As mentioned above, the conventional solution to repairing faulty primary cementing has been to force a cement composition into the voids, cracks or channels formed and to permit the cement to set therein whereby the voids, cracks or channels are sealed. However, when the voids, cracks and channels are very small, it is often difficult to squeeze conventional cement compositions thereinto.
Recently, fine particle size hydraulic cement compositions have been used in the construction and repair of wells. Such fine particle size hydraulic cement compositions and methods of their use are described in detail, for example, in U.S. Pat. Nos. 5,086,850 entitled Squeeze Cementing issued Jun. 16, 1992 and No. 5,125,455 entitled Primary Cementing issued on Jun. 30, 1992. The fine particle size cement compositions and methods have been utilized successfully in primary cementing, squeeze cementing and other forms of well cementing. Because of the fine particle size of the cement, the compositions can readily be forced into and through very small openings, they can have low densities and they quickly develop gel strength after placement. However, problems which have persisted in the use of such fine particle size cement compositions is that they often tend to prematurely gel, have poor compressive strength development and suffer from subsequent loss of compressive strength, a condition known in the art as compressive strength retrogression.
Thus, there remains a need for fine particle size cement compositions and methods wherein the compositions do not prematurely gel and have good compressive strength development without the occurrence of subsequent compressive strength retrogression.